Water activity and water potential are related to each other by equation 1 below:Ψ=(RT/Vw)n(aw)  [1]wherein Ψ is the water potential (Pa), R is the constant of the gases (8.3145 jK−1 mol−1), T is the Kelvin temperature, vw (1.8 10−5 m3) is the water partial molar volume and aw is the activity of water, which can be taken as the hundredth part of the relative humidity of balance with the sample.
The evaluation of water activity is very important, for instance, in food technology and in storing grains. For a large number of these products the storage useful life and the quality itself depends on the water activity, which is used as a control parameter.
To the plant physiology and to the handling of water in the soil, however, the notion of water potential with its subdivisions into components, in general, is more used than the notions of relative humidity and of water activity. The water potential was defined as a relationship between the chemical potential and the partial molar volume of water. As a result, the water potential can be treated simply as force per area unit, that is, as pressure. The water potential thus defined is a sum of pressure components. Among these, in the plant, the following are important: the osmotic component (Ws), with which by accumulation of solutes the cell sucks water into itself; the gravitational potential (H), a height to be overcome for water to move between the root in the soil and the leaves and other aerial organs; the turgescence pressure (Ps) inside the cells, a result from the osmotic potential and from the cell membranes and walls; and the water tension in the xylem and in the apoplasma (PA), in the matrix of which the differences in pressure, modulated by growth and transpiration, govern the velocity of transportation fo this fluid between the soil and each organ of the plant.
The term “tensiometer” may be used to designate instrument for measuring the water-tension component either in the soil or in the plant. Various types tensiometer find applications in branches such as plant physiology, handling irrigation and geology. Instruments for measuring water activity, balance relative moisture and water potential (eq. 1) on the other hand could be called preliminarily hygrometers, and they have applications ranging from the purely practical applications to more sophisticated ones directed to science in areas such as plant physiology and food technology.
Much of the classic technology on construction and uses of tensiometers and hygrometers for studying echophysiology of plants were revised by Slavick (Methods of studying plant water relations, Springer, New York, Springer, 1974, ISBN 0-387-06686-1), and with regard to the water activity in foodstuffs, several frequently used methods are briefly described by Zapata et al. (Bol. SBCTA, v30, n. 1, pp 91-96, 1996).
The ordinary tensiometer (Soil Science, v. 53, pp 241-148, 1942), used for handling irrigation, is constituted by a porous capsule with a cavity filled with water, hermetically connected to a vacuum gauge. Its working ranges from zero pressure to barometric pressure, but in practice it is used mainly between zero and 70 kPa. The most widespread use of the ordinary tensiometer is as a reference instrument in handling irrigation. The great limitation of the ordinary tensiometer, in turn, is the occurrence of embolism, that is, accumulation of air in water contained in the cavity of the porous capsule. This gradual accumulation of air causes the tensiometer to lose response velocity and to undergo a related reduction of the maximum working tension, while the accumulated volume of air increases in the sensor cavity. The maintenance required for the tensiometer to function again is opening the lid, adding water, closing it and await a new response of dynamic balance. This seems to be easy maintenance, but this work has been the great obstacle for the ordinary tensiometer to be used in automation, in the face of the demand for sensors that operate with low or no maintenance.
Module water tension higher than barometric pressure, which rises up to 1500 kPa or higher, can be measured with the tensiometer developed by Ridley & Burland 1993 (Géotechnique, v. 43, pp 321-324, 1993). The principle of this high-performance tensiometer is similar to that of the ordinary tensiometer, from which it differs in that it has a cavity with a reduced volume over a porous element of high bubbling pressure, an electronic pressure transducer and in that it is pre-hydrated to over 4000 kPa for at least 24 hours, in a hyperbaric chamber, in order to dissolve the air bubbles. It is a high-performance instrument in the sense of high water pressures which it measures. However, it is an instable instrument, the operation of which is often interrupted by the occurrence of embolism, after at most a few hours of operation. Embolism in this instrument has a much more devastating and instantaneous effect than that experienced with an ordinary instrument, when the water tensions measured are much higher than the barometric pressure module. In spite of this limitation, this is a valuable system for geophysicists and engineers who need to measure mechanical properties of soils in a wide range of water tensions.
A system of measuring water tension that does not have the embolism limitation is that described in document BR PI 0004264-1. In this system, porous capsules with the cavity filled with air and without water are subjected to air pressure so that the water tension will be measured by difference between the parameter pB (bubbling pressure) and air pressure (p) required to force permeation of the gas through the wall of the porous element. Porous elements with properties suitable for different agricultural applications and different instruments of system use are available commercially.
Most instruments for measuring water tension in the soil are gauged in a pressure chamber with porous membrane or disc, the called Richards chamber (Soil Science, v. 51, pp 377-386, 1941). In this pressure chamber, water is forced out of the equipment through the porous membrane on which the study sample rests, usually the soil. The time of balance of water tension depends on factors, among which are important the thickness of the soil layer and the bubbling pressure of the membrane. The water tension in the balance is equal to the air pressure applied. This instrument has been used mainly to establish curves between the moisture and the water tension in the soil, the so-called retention curves and volume-pressure curves. However, this is a gauging device, not a water-tension sensor proper. As a limitation, the adjustment of high tensions is too slow, because the hydraulic conductivity of the soil decreases exponentially with the water tension.
For preserving grains and various food products, it is indispensable to maintain the water activity that corresponds to matrix potential and total water potentials much higher than 3.0 MPa (aw>0.98). For instance, grains in hygroscopic balance with relative humidity of 50% would have aw=0.50 and water potential of 96 MPa. For gauging these instruments, which measure water activity, techniques of hygroscopic balance with saline solutions and instruments for preparation of reference relative humidity are used. A simple system that makes use of hygroscopic salts for adjusting relative humidity in chamber under controlled temperature was invented by Greaves in 1991 and is described in document GB 2255190. Other international systems accepted for gauging hygrometers are: 1) gaseous mixtures of air from two pressures, one of saturation and the other of measurement, both in isothermal environment; 2) mixtures of air from two temperatures, one of saturation, more reduced, and the other of higher measurement; 3) and the gravimetric system in which a known mass of water is vaporized inside a container of known volume and finely adjusted temperature (PI BR 0104475-3).
Water activity or relative humidity of balance is the main variable related to the preservation of foods (Bol. SBCTA, v. 30, pp 91-96, 1996). The methods for determining water activity in foods are varied and include:                a) Gravimetric methods, which are based on determining the drying (desorption) or moistening (sorption or adsorption) curve of a food or soil during the balance with reference saturated saline solutions in isothermal condition. This is a method the velocity of which decreases rapidly as a result of the increase in the dimensions of the organs, or sample units, and it may take weeks or even months depending on the material. It is a method that needs to be applied at strictly controlled temperature in order to prevent water condensation. In addition, the saturated solutions employed should preferably adjust the water activity in a manner practically independent of temperature, that is, with a small thermal coefficient. Since this method involves a long wait, it can only be used for little perishable foods, as is the case of various seeds.        b) An isopiestic procedure involves strips of filter paper soaked in different saturated reference saline solutions. These strips are weight and placed into the chambers with the product for 24 to 48 hours. Each strip may gain or lose mass depending on whether its water activity is higher or lower than the water activity of the sample. Thus, by using graphic interpolation, one estimates the water activity of the sample in which the strip soaked in an adequate saline solution would not undergo variation in mass. It is a method of good quality and low cost, but it is a procedure that involves “attempts” and is considered a slow method.        c) The balance method with a sensor of an reference absorbent material made of cellulose or casein, for example, involves the preliminary establishment of a calibration curve that relates the sensor mass and the water activity from standardized saline solutions. Then, the sensor of absorbent material is put together with the sample and, after a period of 24 to 48 hours, one determines its final balance mass, taking care to prevent any loss of water until the weighing. The water activity is then calculated in accordance with the sensor mass, by using the calibration curve. This technique functions for desserts and could be used for plant organs with water activity ranging from 0.8 to 0.99. This is a low-cost and interesting method with precision on the order of 0.002 units of water activity. It is also a slow method whose response stability depends on the absorbent material. An important limitation of this technology is that it is not suitable for water potentials close to zero.        d) Fiber hygrometers for measuring relative humidity, as described in document GB 344341 of 1931 are simple and practical. These instruments, however, require frequent calibration, since fibers like those of degreased hair thread, for example, lose their elasticity as time passes. Moreover, they are instruments that have not been built specifically for measuring water activity in foodstuffs.        e) Resistive electronic hygrometers that, in general, are made from a blade of an invert material, coated with a hygroscopic layer of lithium chloride, for example. In these systems the electric conductance varies as a function of the relative humidity of air. The precision of this type of device is on the order of 0.005 unites of aw. Bonne et al, in 1996, developed a stabilized rapid-response microsensor for measurement of absolute units and of dew-point temperature based on the hygroscopicity of lithium chloride, according to description in document U.S. Pat. No. 5,533,393. The present degree of sophistication of these resistance devises may be high, as can be observed in the resistive sensor with a porous element and heating, described in document Speldrich WO2005/121781, in which the electronic heating of the porous element vaporizes condensed water droplets and also enables the determination of relative humidity higher than 100%. The response of the electrical resistance devices, however, tends deteriorate, for instance, by dilution or by accumulation of ions on the inert matrix.        f) A second category of electronic hygrometers are those of capacitance, which make use of the high dielectric constant of water vapor, in comparison with air. One of these systems with aggregation of complexity for improvement of the precision is described in document U.S. Pat. No. 5,922,939. In general, these are absolute-humidity sensors that respond rapidly and that require thermal corrections for measurement of relative humidity. Additionally, they tend to be little sensitive in measurements of relative humidity close to saturation. Capacitive sensors, just as resistive sensors, in general, require frequent calibration.        g) Dew-point temperature method for measuring the water activity, in general, requires measurement of temperature over a thermocouple surface or a cooled mirror surface, for example. The dew-point systems can also produce wrong estimates of the water activity, if the cooled surface is contaminated by impurities, and also in the case presence of volatile substances. In general, the instruments based on dew-point temperature are more stable and less subject to interference than electrical resistance hygrometers and those of capacitance.        h) In the psychometric method the temperature of a thermometer with humid bulb and the temperature of the dry-bulb thermometer, of reference, are used to estimate the water activity. Usually, the humid bulb is humidified thanks to the previous condensation of water onto a cooled surface by application of electric current (Peltier effect).        
Among the instruments for measuring water activity, those that have been considered the best ones are those that use the dew-point method, according to the temperature at which the condensation of water on a clean and hydrophilic surface is started. Campbell, in 1974, developed a hygrometer based on thermocouples and the Pelier effect for measuring osmolality, water activity, or water potential of solutions and plant samples. The device, the electronics and the methodology for using the instrument at the drew point and in the psychometer mode of humid bulb are described in document U.S. Pat. No. 3,797,312. One of the qualities of the equipment described is the portability and the fact that it does not require a sophisticated thermal bath system for measuring water activity in small samples of plant and of solution. In the psychometric mode the temperature of the humid bulb formed after condensation of water and the temperature of the dry bulb of reference are used for estimating water activity. Other devices that use the dew-point method make use of a mirror as in the case of the device of Zlochin (2005), described in document U.S. Pat. No. 6,926,439, in which the mirror always free from impurities brought by air is used for application of the dew-point method. Zlochin argues that one of the great problems of the dew-point method is the accumulation of impurities carried by air. The removal of these impurities is difficult, because there is a demand for frequent cleaning the cooled surface, so that the quality of the measurements will not be impaired.
According to Campbell & Lews (1998), in the system for measuring water activity by the dew-point method of document U.S. Pat. No. 5,816,704, and in other devices that make use of this principle, the error in estimating the dew-point temperature is given by the equation: Erro2=2 r a w/s, wherein r is the resistance of the laminar layer (s m−1), w is the inclination between the saturation vapor pressure and the temperature, w is the condensation rate in g m−2 s−1, and s is the concentration of saturation vapor in g m−3. From this equation it becomes evident that methods that decrease the laminar layer and the amounts of condensed water for the measurement also diminishes the error in estimating the dew-point temperature and increase the response velocity of the instrument.
Unlike the case of soils, considered initially, the development of devices for measuring water tension in plants has been more rare, in spite of being a fundamental variable to explain the rising of the sap. In the prior art the most widely used method for measuring water tension in plants have been the Scholander pressure chamber (Proceedings National Academy of Sciences USA, v. 52, p. 119-125, 1964). For use, a leaf, for example, is secured to the orifice of the sealing rubber, so that the petiole can go through the cover that closes the chamber hermetically. Upon measuring, the gas pressure in the chamber increases slowly until the first sap drop pours through the petiole. The gas pressure applied, in this condition, is then taken as stimulative of the water tension in the leaf. In the current literature, however, there are controversial positions on the validity of this method, the weak point of which is that it does not have a gauging form. In spite of this, and even involving destructive samplings, the Scholander pressure chamber is the most widely used instrument for studying water relations in plants.
The water tension in a plant has also been measured by inserting a capillary tube into the xylem pots (Plant Physiology, v. 61, pp 158-163, 1978). However, this method, called pressure probe method, is extremely difficult, laboratorial and has not enabled measurements of tensions higher than 800 kPa in a plant. In the comparative measurements of water tension in plants by using the pressure probe and the Scholander pressure chamber, additionally the results have not always been equivalent, within the margins of error.
In the present invention, one describes a simple method for measuring water potential, water tension and water activity, which can be gauged through calibration by different procedures. In measurements of water tension, the system of this invention is not subject to embolism problems, a typical problem of a tensiometer. In order to measure water potential, the system may present a rapid thermal balance, using a contact microchamber. In measurements of water tension in the soil, it enables readings by simply using a sliding gauge in the range from 0 to 0.3 MPa. The water potential may be measured with a microscope between zero and 3.0 MPa. In plants, additionally, under a microscope, the water tension measured by putting the sensor against the sample by flattening, is on the same order as the measurement of water potential.